I Love Heathkit! T-3

Not long after I discovered Mr. Carlson’s lab on Youtube he did a full restore on the first bit of Heathkit gear I’d ever seen

It was the model T-3 signal tracer.

It was beautiful
I was hooked
It looks like this…

When one appeared on E-Bay I simply had to have it… so I did. Mine has had a pilot light added – it’s not official & wrecks the symmetry of the controls but at least the modder used an appropriate jewel which was popular with Heathkit at the time. It’s also nice to know when it’s on
I’m very glad I watched the whole Carlson video – the circuit was released with an error in the grid circuit for the driver – in this case a 12SH7. Have a look at how the volume control is connected to the grid…

The pot is the grid leak so altering the volume also varies the bias to G1 - this puts DC on the carbon track. Result = crackle & an unhappy valve. There’s a simple mod to fix this:
A 5.6 meg resistor from G1 to ground;
A .01 capacitor between the wiper & the G1/5.6Meg junction.
Works well!

Here’s the underside of my beastie as received.

Construction quality wasn’t too bad – some of the wiring had insulation stripped a bit too much. Also, at the bottom left hand side one of the filter cap positives tags is 1mm away from an earth tag. This is one thing to remember about Heathkits – they were all put together by enthusiasts so don’t expect a perfect wiring/soldering job.

My usual course of action is to replace all wax caps & the main filter can. If I have time I check leakage on all – it’s a rare wax cap that doesn’t leak after 70-odd years. If they are any small electro’s, give them a leakage test, an ESR test, a value test & a careful visual inspection. If in doubt, replace these too. When it came to the filter can I decided to treat myself with a new one from AES in the good ‘ole USA. They make an exact replacement.

Check all resistors – some will be high. Replace as needed.

Input connectors – have another look at the first pic. The high gain channel connector is what I was brought up to call an Amphenol connector. Amphenol made more than just this type though. & the plugs are very expensive. The low gain channel simply uses a mon 1/4-inch jack & socket. Mr. Carlson recommended replacing both with BNC sockets. I have no problem with this action Vs. originality… I use this instrument quite a bit & I don’t think BNC sockets look too much out of place.

I should point out that the RF socket is not demodulated. I think Heathkit assumed that a diode probe would be used here. This socket connects to the grid of the 12C8 via a capacitor which provides useful gain when searching for signals. The Hi Gain switch operates here – it grounds the 12C8 grid in the off position.

The Audio socket mixes with the 12C8 output into a 12SH7. The signal then passes to the 12A6 & then to an o/p transformer & speaker. All contact points around this area appear on five terminals on the front panel. Just watch out for the ones on the primary side – they all have full HT on them although the shrouding seems quite satisfactory. It is possible to trace using the magic eye only – there’s a speaker off-on switch provided.